# setjmp

From cppreference.com
< c‎ | program

Defined in header ``<setjmp.h>`` |  |
#define setjmp(env) /* implementation-defined */ |  |

Saves the current execution context into a variable `env` of type jmp_buf. This variable can later be used to restore the current execution context by longjmp function. That is, when a call to longjmp function is made, the execution continues at the particular call site that constructed the jmp_buf variable passed to longjmp. In that case `setjmp` returns the value passed to longjmp.

The invocation of `setjmp` must appear only in one of the following contexts:

1. The entire controlling expression of `if`, `switch`, `while`, `do-while`, `for`.

       switch(setjmp(env)) { // ...

2. One operand of a relational or equality operator with the other operand an integer constant expression, with the resulting expression being the entire controlling expression of `if`, `switch`, `while`, `do-while`, `for`.

       if(setjmp(env) > 10) { // ...

3. The operand of a unary ! operator with the resulting expression being the entire controlling expression of `if`, `switch`, `while`, `do-while`, `for`.

       while(!setjmp(env)) { // ...

4. The entire expression of an expression statement (possibly cast to `void`).

       setjmp(env);

If `setjmp` appears in any other context, the behavior is undefined.

Upon return to the scope of `setjmp`:

- all accessible objects, floating-point status flags, and other components of the abstract machine have the same values as they had when longjmp was executed,
- except for the non-`volatile` local variables in the function containing the invocation of `setjmp`, whose values are indeterminate if they have been changed since the `setjmp` invocation.

### Parameters

env | - | variable to save the execution state of the program to.

### Return value

​0​ if the macro was called by the original code and the execution context was saved to `env`.

Non-zero value if a non-local jump was just performed. The return value is the same as passed to longjmp.

### Notes

Above requirements forbid using return value of `setjmp` in data flow (e.g. to initialize or assign an object with it). The return value can only be either used in control flow or discarded.

### Example

Run this code

    #include <stdio.h>
    #include <setjmp.h>
    #include <stdnoreturn.h>

    jmp_buf my_jump_buffer;

    noreturn void foo(int status)
    {
        printf("foo(%d) called\n", status);
        longjmp(my_jump_buffer, status + 1); // will return status+1 out of setjmp
    }

    int main(void)
    {
        volatile int count = 0; // modified local vars in setjmp scope must be volatile
        if (setjmp(my_jump_buffer) != 5) // compare against constant in an if
            foo(++count);
    }

Output:

    foo(1) called
    foo(2) called
    foo(3) called
    foo(4) called

### References

- C17 standard (ISO/IEC 9899:2018):

  - 7.13.1.1 The setjmp macro (p: 191)

- C11 standard (ISO/IEC 9899:2011):

  - 7.13.1.1 The setjmp macro (p: 262-263)

- C99 standard (ISO/IEC 9899:1999):

  - 7.13.1.1 The setjmp macro (p: 243-244)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.6.1 The setjmp macro

### See also

longjmp | jumps to specified location (function)
C++ documentation for setjmp

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=c/program/setjmp&oldid=148503"